Volume reflection holograms have been mass-produced in the past, predominantly in monochromatic form, by companies such as Applied Holographics plc, Third Dimension Ltd., and Du Pont Authentication Systems inc., by a process of contact copying of image-planed master holograms. These master holograms have generally been created by one of two alternative techniques.
One technique involves the production of second-generation (H2) contact masters from redundant first generation (H1) recordings where the recording of subject matter distant to the recording plate enables multiple channels of image information to be recorded in the H1 such that image-switching effects occur when the second generation hologram is viewed by a viewer's eye, effectively through the window of the virtual H1. This technique is commonly used in holography and is similar to the method used to create classical embossed hologram masters. It is referred to as ‘conventional H1/H2 mastering’, or ‘split beam holography’ and follows the published work by Upatnieks and Leith in the 1960's.
A second technique allows for the initial recording of a first generation master hologram especially in a medium such as dichromated gelatin, whose grainless clarity, low absorption, and high maximum diffraction efficiency capability enable the preparation of a near-field recording with exceptional brightness and a very wide window of view. This type of hologram where a hologram recording of an image from an object or a master hologram is made with the use of only one expanded laser beam, such that the single beam acts both as the reference beam for the hologram and also gives rise to the object beam when reflected back from an object or hologram, is called a ‘Denisyuk’ or ‘single-beam’ hologram.
The first of these techniques leads to bright multi-channel images which have been criticised by some observers for their limited viewing angle, equivalent to a window of the same size as the H1 master, spaced some distance from the final hologram.
The latter method, however, provides a very wide viewing angle since a master, typically of the same size, may be placed in this case very close to the subject matter. Thus the viewing window of this system may be equivalent to almost a complete hemisphere. However this type of master does not possess the capability to provide animation, or image switching, and because light is diffracted into such a wide viewing zone by this technique the image brightness as perceived from any particular viewing position will tend to be less than with the former method of mastering.
It is nonetheless desirable to be able to record multiple different holographic images in a single recording medium, especially for security applications. It is also desirable to be able to record images onto flexible film rather than rigid glass plates for many reasons including cost and ease of mass production. However the flexible nature of film provides some special challenges which can make recording of holographic images, especially multiple images for security purposes, difficult in practice.
One specific problem associated with recording holograms into film as compared with a glass substrate arises because typically the film must be held substantially motionless (say to better than a quarter wavelength) for a relatively long period whilst the hologram is recorded. This is difficult because film is not rigid. A further problem arises because the hologram emulsion tends to be cloudy, especially towards the blue end of the spectrum, and this can also make the recording of multiple different holographic images into a single film hologram difficult.